Apparatus for analysis of mixed gas components

ABSTRACT

The present invention provides an apparatus for the analysis of mixed gas components which can perform, in high precision, determination of the . quantities of components contained in a sample gas containing a plurality of the components having molecular weights close to each other and which has a Fourier transform mass spectrometric means for ionizing a sample gas, applying a high frequency electric field to the ionized gas to induce cyclotron resonance, detecting the cyclotron resonance as a high-frequency decaying electric signal, and converting the resulting high-frequency decaying electric signal to a frequency-domain signal and a wavelength variable light irradiating means for irradiating a light of a single wavelength to ionize the molecules of the components constituting the sample gas, said irradiating means being able to vary the wavelength and/or intensity of the irradiation light.

This application is a division of Ser. No. 08/721,524 filed Sep. 26,1996 now Pat. No. 5,777,205.

TECHNICAL FIELD

The present invention relates to an apparatus for the analysis of mixedgas components which can separate the respective components constitutinga sample gas which is a mixed gas, identify the components and determinethe quantities of them in a short time.

PRIOR ART

Industrially, there are a lot of necessities to separate individuallythe components constituting a mixed gas and determine the quantities ofthem. The analysis of mixed gases are carried out in various fields,particularly, in the fields of industrial analysis, for example,analysis of exhaust gas from automobiles, analysis of reaction gas inchemical industry processes and control of reaction process, analysis ofrespiratory gases for knowing the process of medical treatment andbiological reaction. It becomes necessary to separate the componentsindividually and determine the quantities thereof in the analysis.Besides, in many of these cases, rapid analysis is required.

For example, in the exhaust gas analysis, a rapidness of the order ofmillisecond to second is desired as a measuring period. In therespiratory gas analysis, it is required that measurement data at tenpoints or more can be obtained during one respiration cycle (3-4seconds). In a process of chemical industries, a longer measuring periodis allowed, but, even in this case, if the analysis period is shorter,more times of measurement can be performed in the same measuring time.If more times of measurement can be conducted in a given measuring time,analytcal accuracy can be improved by the known time averaging method.In the field of industrial analysis, there have hitherto been oftenemployed those methods which use, in combination, various apparatus forthe analysis of the various sample components respectively, such ashydrogen, water, ammonia, oxygen, hydrocarbons, etc.

However, although the use of a combination of these exclusive analyticalapparatus may be sometimes advantageous for the improvement ofsensitivity in that the physical or chemical properties peculiar to therespective sample components are measured, a plurality of analyticalapparatus differing in principles depending on the kinds of thecomponents must be provided. Besides, if different components having thesame properties are contained, responses corresponding to the saidcomponents overlap in the output of the analytical apparatus, and theresponses of the respective components cannot be discriminated from eachother. Presence of such interfering components causes unexpected errorsin measurement.

Generally, in these industrial analyses, it is desired essentially thatsimultaneous analysis of many components can be performed in a realtime. In order to satisfy the demand for simultaneous measurement ofmany components, the conventional annalytical apparatus based on thedifference in chemical properties of the components is limited todealing with such components as having similar chemical properties, andthere have been inevitable difficulties in multi-component analysis.Therefore, for this purpose, physical measurement, so-calledinstrumental analysis, in which the differences of physical propertiespossessed by any materials in common being measured, are desirable.Spectrometric apparatuses such as gas chromatograph (hereinafterreferred to "GC") and infrared spectrometer (hereinafter referred to"IR") have been conventionally used for separation and determination ofcomponents of a mixed gas. However, as for the GC method, owing to itsoperational principle that the respective components are separated withlapse of time on the basis of the difference in retention time of therespective components passing through the separation column, there areserious problems in that the setting of measuring conditions, such asthe selection of the column for the respective components to beanalyzed, the determination of the temperature condition in the oven inwhich the column is put, or the setting of flow rate of the carrier gasare complicated, and that depending on the number or the kind of thecomponents, a complex means of multi-dimensional column is needed fromthe viewpoint of hardware and rich experience and high skill are neededfrom the viewpoint of software. Furthermore, some of the differentcomponents can be equal or nearly equal in the retention time, and,naturally, separation of these components from each other is impossibleor difficult. In addition, as well known, the GC method requires a longtime of several minutes to several ten minutes as a measuring period,and this method cannot be applied to the above-mentioned high-speedanalysis.

In direct analysis of a mixed gas by an infrared spectrometer, in manycases, the absorption lines corresponding to the respective componentsoverlap each other and a resolution sufficient to distinguish a spectrumof a component from that of the other cannot be obtained. Furthermore,components having no infrared absorption, such as nitrogen, oxygen,chlorine and hydrogen cannot be detected. Sulfur dioxide, carbondioxide, water and the like produce interference between spectra. Thus,in general, the discrimination of the components of a mixed gas isconsiderably difficult.

Ultraviolet spectroscopy and other spectroscopic analyses have the sameproblems as the above in general-purpose use.

The analytical principle of mass spectrometer is to ionize gas moleculesintroduced into a vacuum chamber and discriminate the gas components bythe ratio of mass and charge. Therefore, there are substantially noundetectable gas components and the mass spectrometry can be said to bean analytical means which has the highest general-purpose usability inmulti-component analysis.

However, regarding gas components equal in the mass number, spectrumpeaks thereof overlap and the discrimination of the components isgenerally difficult. Conventionally employed methods in such case are asfollows:

(a) collating the mass spectra of the component gases with each otherand selecting the peaks which have no overlapping in the spectra(hereinafter, referred to unipeaks) to perform the discrimination;

(b) discriminating the components from group of component peaks presenttogether by multiple regression analysis;

(c) connecting a gas chromatograph column in front of a massspectrometer, whereby a mixed gas is separated into pure components, andidentifying and determining the respective pure components eluted fromthe column in succession by a mass spectrometer;

(d) measuring the mass of component peak in a high resolution, i.e. anability of separating resonance lines of samples having close molecularweights and being defined to be "molecular weight/full width at halfheight", of 10³ -10⁴ which enables the detection of the mass defect ofatoms constituting a molecule by using a high resolution massspectrometer, and thus, obtaining the chemical composition of thecomponents;

The method of (a) lacks general applicability because the unipeakdiffers depending on the component composition of the sample gas and,furthermore, proper peaks are not always found.

The method of (b) can be applied under the condition that mass spectraof all of the components of the mixed gas are known and patterncoefficient thereof have been correctly obtained. When unknown componentis contained, there is a disadvantage that an unexpectedly great erroroccurs.

The method of (c) has the same problems as of the gas chromatograph,such as a long measuring period.

The method (d) is a method for obtaining chemical composition of the gascomponents by accurately measuring the mass of component ion, which ispossible only by using a large double-forcusing mass spectrometer havinga high resolution or Fourier transform ion cyclotron resonance massspectrometer (hereinafter, referred to "FT-ICR"). Especially, FT-ICRmethod has a great feature that measurement can be performed rapidly ina short time of from about several ten milliseconds to the order ofsecond, however, a double-forcusing mass spectrometer is so large thatit is not suitable for installing at an industrial analysis site, andtherefore, there have been no examples of using it for industrialanalysis.

Components having the same chemical composition cannot be separated evenby a FT-ICR. For example, all of isobutene, 1-butene and 2-butene havenot only a mass number of 56 but also a chemical composition of C₄ H₈,and, hence, they have utterly the same molecular weight of 56.06260.Therefore, it is impossible even with a high resolution of 10⁴ -10⁵ orhigher of FT-ICR to carry out separation and determination of thesecomponents only by accurate measurement of the mass. This brings aboutserious problems in rapid analysis of a mixed gas rich in hydrocarbonscontaining various isomers as in the analysis of exhaust gas frommotorcar engines.

The object of the present invention is to provide a new apparatus forthe analysis of mixed gas components which can perform separation anddetermination of quantities of not only a plurality of componentsdiffering in molecular weight, but also isomers of the same molecularweight such as the above-mentioned hydrocarbons.

Another object of the present invention is to provide an apparatus forthe analysis of mixed gas components which can perform determination ofquantities, with high accuracy, of components in a sample gas which is amixture of a plurality of components which are equal in molecularweight, but differ in ionization potential.

Still another object of the present invention is to provide an apparatusfor the analysis of mixed gas components which can perform determinationof quantities, with high accuracy, of components in a sample gas whichis a mixture of a plurality of molecules which are equal or near inionization potential, but differ in molecular weight.

DISCLOSURE OF INVENTION

The present invention which aims at accomplishment of the above objectsis an apparatus for the analysis of mixed gas components, characterizedby comprising a Fourier transform mass spectrometric means which ionizesa sample gas introduced into a high-vacuum cell placed in a staticmagnetic field, applies a high-frequency electric field to the ion byapplying a high-frequency to a pair of irradiation electrodes providedin the high-vacuum cell, induces an ion cyclotron resonance for the ionof a specific component to be measured, detects said ion cyclotronresonance as a high-frequency decaying electric signal, converts thehigh-frequency decaying electric signal to a digital signal, andconverts the digital high-frequency decaying electric signal which is atime-domain signal to a frequency-domain signal, and a variablewavelength light irradiating means which irradiates said sample gas witha light of a single wavelength to impart an ionization energy to themolecule of the components constituting the sample gas, the wavelengthof the light irradiated to the sample gas being able to be varied.

The apparatus for the analysis of mixed gas components of the presentinvention will be explained below.

The apparatus for the analysis of mixed gas components of the presentinvention has a Fourier transform mass spectrometric means which ionizesa sample gas, applies a high-frequency electric field to the ion byapplying a high-frequency to a pair of irradiation electrodes providedin a high-vacuum cell to induce an ion cyclotron resonance for the ionof a specific component to be measured, detects said ion cyclotronresonance as a high-frequency decaying electric signal, converts thehigh-frequency decaying electric signal to a digital signal, andconverts the digital high-frequency decaying electric signal which is atime-domain signal to a frequency-domain signal, and a wavelengthvariable light irradiation means which irradiates said sample gas with alight of a single wavelength which ionizes the molecule of thecomponents constituting the sample gas and is constituted so that thewave length of irradiation light can be continuously varied.

In this apparatus for the analysis of mixed gas components, moleculeshaving an ionization potential lower than the energy level of anirradiation light having a given wavelength are ionized, while moleculeshaving an ionization potential higher than the energy level of theirradiation light are not ionized and removed from the measuring systemas neutral gas molecules. That is, mass spectra free from overlapping ofthe spectra of unnecessary ions are obtained by selective ionization,and, furthermore, determination of a spectrum peak becomes possible bythe high mass resolution which is a feature of the Fourier transformmass spectrometric means.

Another embodiment of the apparatus for the analysis of mixed gascomponents of the present invention comprises a Fourier transform massspectrometric means which applies a high-frequency electric field to ionby applying a high-frequency to a pair of irradiation electrodesprovided in a high-vacuum cell to induce an ion cyclotron resonance forthe ion of a specific component to be measured, detects said ioncyclotron resonance as a high-frequency decaying electric signal,converts this high-frequency decaying electric signal to a digitalsignal, and converts the digital high-frequency decaying electric signalwhich is a time-domain signal to a frequency-domain signal, a variablewavelength light irradiating means which irradiates said sample gas witha light of a single wavelength which ionizes the components constitutingthe sample gas and is constituted so that the wavelength of theirradiation light can be varied, and a subtraction processing meanswhich subjects to subtraction processing a first mass spectrum detectedby irradiating the sample gas with an irradiation light of a givenwavelength and a second mass spectrum detected by irradiating the samplegas with an irradiation light of a given wavelength which differs fromthe wavelength of the first irradiation light.

In this apparatus for the analysis of mixed gas components, thecomponent having an ionization potential lower than the energy level ofan irradiation light having a given wavelength is ionized by irradiatingthe sample gas with a light having a given wavelength by the variablewavelength light irradiating means and the component having anionization potential higher than the energy level of irradiation lightis not ionized and removed from the measuring system as neutral gasmolecules. That is, ionization is selective. Overlapping of unnecessaryions is avoided by this selective ionization, and the first massspectrum of high precision is obtained by the Fourier transform massspectrometric means. Subsequently, the sample gas is irradiated with alight of wavelength different from that of the light irradiated before,thereby to ionize the component having an ionization potential lowerthan the energy level of the irradiation light of the above wavelength,and the component having an ionization potential higher than the energylevel of the irradiation light is not ionized and removed from themeasuring system as neutral gas molecules. Overlapping of any spectra ofunnecessary ions is avoided by this selective ionization, and the secondmass spectrum of high precision is obtained by the Fourier transformmass spectrometric means. The first mass spectrum and the second massspectrum are subjected to subtraction processing by the subtractionprocessing means to obtain a difference spectrum. The resultingdifference spectrum is a mass spectrum of only the component having anionization potential equal to the gap between the energy level of thefirst irradiation light and that of the second irradiation light. Byselecting the respective wavelengths of the first and second irradiationlights, it becomes possible to freely and selectively obtain massspectra of any desired gas components even of a sample gas which is amixed gas showing complex mixed spectrum, and it becomes possible todetermine spectrum peaks of any components.

Another embodiment of the apparatus for the analysis of mixed gascomponents of the present invention comprises a Fourier transform massspectrometric means which applies a high-frequency electric field to ionby applying a high-frequency to a pair of irradiation electrodesprovided in a high-vacuum cell to induce an ion cyclotron resonance forthe ion of a specific component to be measured, detects said ioncyclotron resonance as a high-frequency decaying electric signal,converts this high-frequency decaying electric signal to a digitalsignal, and converts the digital high-frequency decaying electric signalwhich is a time-domain signal to a frequency-domain signal, and awavelength variable light irradiation means which irradiates the samplegas with a light of a single wavelength which ionizes the componentsconstituting the sample gas, and is constituted so that the wavelengthof the irradiation light can be continuously varied and that theluminous intensity of the irradiation light can be freely adjusted.

In the analysis of mixed gas components by this apparatus, byappropriately adjusting the luminous intensity of the first irradiationlight and that of the second irradiation light, fragment peaks can bediminished or removed from a spectrum peak of a component and a massspectrum consisting of only a molecule peak can be obtained. Thus, massspectrum peaks can be easily identified or determined by the adjustmentof luminous intensity.

The apparatus for the analysis of mixed gas components of the presentinvention will be further explained below.

The Fourier transform mass spectrometric means in the apparatus for theanalysis of mixed gas components of the present invention can be of anyconstructions which can realize the functions of applying ahigh-frequency electric field to ion by applying a high frequency to apair of irradiation electrodes provided in a high-vacuum cell to inducean ion cyclotron resonance for the ion of a specific component to bemeasured, detecting said ion cyclotron resonance as a high-frequencydecaying electric signal, converting this high-frequency decayingelectric signal to a digital signal, and Fourier transforming thedigital high-frequency decaying electric signal which is a time-domainsignal to a frequency-domain signal. A suitable example of the apparatushas the following means:

(1) a magnetic field applying means which applies a static magneticfields to ions to induce an ion cyclotron movement for the ions,

(2) a high-vacuum means containing an analysis cell into which a samplegas is introduced and in which molecules of the components constitutingthe sample gas are ionized,

(3) an electronic circuit devise which causes resonance-excitation ofthe ions in said analysis cell and detects and amplifies an inductioncurrent induced in a receiving electrode by the movement of the ions,and

(4) a controlling-operating means which carries out various operationssuch as setting a condition for measurement, Fourier transformation andothers.

The variable wavelength light irradiating means in the apparatus for theanalysis of mixed gas components of the present invention is designed sothat it can impart an ionization energy to the component moleculesconstituting the sample gas and the wavelength of the light to beirradiated to the sample gas, i.e. irradiation light, can becontinuously varied and further, the luminous intensity of saidirradiation light can be changed.

The above-mentioned magnetic field applying means is a mechanism whichinduces an ion cyclotron movement for the ions present in the analysiscell, and is provided with, for example, a magnet which applies a staticmagnetic field to the analysis cell, and, preferably, further providedwith a magnetic field generating means having a magnetic fieldcorrecting coil. In this magnetic field applying means, preferred is amagnetic circuit which comprises a pair of magnets arranged opposite toeach other so that there is provided a space sufficient to place theanalysis cell between them, a supporting member which holds the pair ofthe magnets to support them, and pole pieces provided on magnetic polesurface of the pair of the magnets. In the magnetic circuit having suchstructure, leakage flux can be reduced very much, and utilization ofhomogeneous magnetic field space at the center of the gap between themagnetic poles becomes easy because the space between the supportingmeans is in the opened state. Further, a space of homogeneous magneticfield can be obtained by designing the opposing faces of the pole piecesin a proper shape. In other words, preferably, the opposing faces of thepole pieces are designed in such a shape that uniformity of magneticfield distribution can be improved. The magnet which constitutes themagnetic field applying means may be either a permanent magnet or anelectromagnet if a resolution of at least about 10⁴ can be obtained.Among a permanent magnet and an electromagnet, a permanent magnet issuitable from the point of easiness in installation and maintenance.

The above-mentioned high-vacuum means is a device for maintaining a highvacuum in the analysis cell and retaining ions in the cell over a longperiod of time. For example, it has an analysis cell having a space inwhich a sample gas is allowed to be present, a vacuum chamber containingthe analysis cell, a sample gas introducing means for introducing asample gas into the analysis cell and an evacuation means for evacuatingthe analysis cell and the vacuum chamber to a high vacuum. In order toobtain a high resolution, the life of the ions present in the analysiscell must be at least 100 ms, and for this purpose, the pressure insidethe analysis cell is preferably at a high vacuum of about 10⁻⁷ Pa.Therefore, it is preferred to design the vacuum chamber so that insideof the vacuum chamber which contains the analysis cell becomes highlyvacuum. Further, in order to accomplish such a high vacuum, it ispreferred to combine some of evacuation pumps, and suitable is acombination of an oil-free type turbo molecular pump, a molecular dragpump and a diaphragm pump which are connected in tandem.

The analysis cell can be formed of three pairs of electrode groupscomprising trap electrodes, irradiation electrodes and receivingelectrodes to ionize the sample gas in the cell and cause ion cyclotronmovement of the ion. The analysis cell may be a cylindrical analysiscell which comprises a cylindrical body having a center axis along thedirection of the magnetic field with the side wall being divided to fourequal parts and plates provided at both ends of the cylindrical body.Furthermore, a hexahedral analysis cell can be employed where said threepairs of electrodes are three pairs of parallel electrodes which crossat right angles.

The wavelength variable light irradiating means is formed so that alight capable of ionizing the molecules in the sample gas in theanalysis cell can be irradiated and the wavelength of the irradiationlight can be continuously varied.

A suitable wavelength variable light irradiating means has a structurebased on multi-photon ionization method (hereinafter, referred to MPI).One of the features of this MPI method is that the kind of moleculeionized can be selected by the wavelength of the irradiation light. Thisfeature is based on the phenomenon that since each gas molecule has itspeculiar ionization potential, a molecule having an ionization potentiallower than hν is ionized and a molecule having an ionization potentialhigher than hν is hardly ionized, wherein the frequency of theirradiation light is ν and h is Planck constant. Accordingly, the lightirradiated to the sample gas is desirably a coherent light of a singlefrequency.

According to the principle of MPI, there are three modes, namely,non-resonant MPI (harerinafter, referred to NRMPI), resonant two-photonionization (hereinafter, referred to R2PI) and two photon resonantionization (hereinafter, referred to TPRI) for the ionization of gasmolecules by excitation.

In the case of the non-resonant MPI, molecules are immediately excitedto ionization potential from ground state by instantaneously applyingenergy of many photons to the molecules. Therefore, the variablewavelength light irradiating means which employs the non-resonant MPIneeds a high energy laser beam emitting means. In other words, when thisnon-resonant MPI is employed, it is necessary to irradiate a laser beamof high energy level, namely, of short wavelength. That is, a laseremitter extending over the far ultraviolet region may sometimes beneeded.

In R2PI, some photon excites the molecule from ground state to anintermediate state, i.e. a level present between the ground state andthe ionization potential. This intermediate state is a metastable state,and the excited molecules return to the ground state from theintermediate state at an attenuation factor β. Thus, when the number ofmolecules excited to the intermediate state is greater than that ofmolecules returning to the ground state at an attenuation factor β, mostof the gas molecules are excited to the intermediate state byirradiation light by properly increasing the luminous intensity, i.e.intensity of light, of the irradiation light, during which when thesecond photon is irradiated, the molecules are further excited andobtain an energy higher than the ionization potential of the moleculesto become ions. Therefore, even when a light of an energy level lowerthan the ionization potential of the molecules is irradiated, themolecules are efficiently ionized.

In TPRI, two or more photon energy is nearly simultaneously given to themolecule to excite the molecule from the ground state to theintermediate state. In this process, ionization efficiency is lower thanin R2PI. When TPRI is employed, it suffice to employ a low level laser,namely, a long wavelength laser, but the power thereof must beincreased.

In the present invention, any methods of non-resonant MPI, resonanttwo-photon ionization and two photon resonant ionization can be employedand a suitable method is selected depending on the desired analysis,taking into consideration the characteristics of the respective methods.

In the present invention, a suitable light irradiating means usually hasa means for irradiating the sample gas in the analysis cell with acoherent light of simple frequency, preferably has a laser beam emittingmeans which can emit a laser beam having substantially a simplefrequency, such as ultraviolet laser beam. For example, whenhydrocarbons are to be analyzed, the wavelength of laser beam to beirradiated should be 200-400 nm in R2PI since the ionization potentialsof hydrocarbons are mostly in 7-12 eV.

By the irradiation of a coherent light having a single frequency to asample gas, mass spectra of the respective components of differentionization potential contained in said gas sample can be obtained.

Another preferable variable wavelength light irradiating means has saidlaser beam emitting means and a luminous intensity varying means, i.e.light intensity varying means, for varying the irradiation luminousintensity of the laser beam emitted by said laser beam emitting means.When a variable wavelength light irradiating means having the luminousintensity varying means is employed, the proportion of fragment ion andmolecular ion can be controlled by varying the irradiationluminousintensity of the laser beam and, thus, control ofso-called soft/hardionization becomes possible. By irradiating a sample gas containingcomponents which have identical or close ionization potential, butdiffer in molecular weight with a light having suitably selectedintensity, a mass spectrum in which molecule peaks of the respectivecomponents clearly appear can be obtained.

A suitable apparatus for the analysis of mixed gas components has awavelength varying means which varies wavelength of the light from theabove-mentioned variable wavelength light irradiating means. As suitableexamples of the wavelength varying means, variable wavelength laserssuch as dye laser pumped by YAG laser can be mentioned. When a dye laseris employed, since the wavelength of the emitted light is too long toobtain a necessary energy level, it is desirable to provide a device forconverting the frequency of said emitted light to its harmonicfrequency, for example, a frequency doubler or tripler. If thewavelength varying means is provided, the sample gas can be irradiatedwith a light having different wavelength each time, and a mass spectrumof a specific component can be obtained from the mass spectra obtainedby the respective irradiations.

The aforementioned electronic circuit device has a function ofsubjecting the ion in the analysis cell to resonant excitation,detecting a signal and amplifying the detected signal. This electroniccircuit device has, for example, a high-frequency transmitting meanswhich transmits a high-frequency to the irradiation electrode in theanalysis cell and a receiving means which processes the signal receivedby the receiving electrode in the analysis cell.

The controlling-operating means performs measurement controlling andvarious operations such as Fourier transformation. A suitablecontrolling-operating means has a Fourier transformation means forconverting to a frequency-domain signal a high-frequency decaying signaldetected by a receiving means such as a receiving electrode fordetecting an ion cyclotron resonance signal derived in the analysis celland amplified to convert it to a digital signal, a memory means formemorizing a mass spectrum which is a frequency-domain signal obtainedby the Fourier transformation means, and a subtraction processing meansfor subtraction processing wherein a substraction is carried out betweenspecific mass spectra, i.e. the first mass spectrum, obtained byirradiating the sample gas with an irradiation light of a specificwavelength and memorized by the memory means, and another mass spectrum,i.e. the second masspectrum, obtained by irradiating the sample gas withan irradiation light of a wavelength slightly differing from the saidspecific wavelength and memorized by the memory means. By the apparatusfor the analysis of mixed gas components having the variable wavelengthlight irradiating means provided with a wavelength varying means and thecontrolling-operating means provided with a memory means and asubtraction processing means, the respective components in a sample gascontaining a plurality of components having the same or nearly the samemolecular weight, but differing inionization potential can be identifiedand quantitated. Furthermore, by the apparatus for the analysis of mixedgas components having the variable wavelength light irradiating meansprovided with a wavelength varying means and a luminous intensityvarying means and the controlling-operating means provided with a memorymeans and a subtraction processing means, the respective components in asample gas containing a plurality of components having the same ornearly the same molecular weight, but differing in ionization potentialcan be separated and quantitated. The respective components in a samplegas containing a plurality of components differing in the molecularweight, but having the same or nearly the same ionization potential alsocan be separated and quantitated by the above-mentioned apparatus.

The apparatus for the analysis of mixed gas components of the presentinvention is used in the following manner.

A sample gas is introduced into an analysis cell placed in a highvacuum. A static magnetic field is applied in the analysis cell. Thesample gas in the analysis cell is irradiated with a light of a specificwavelength by the light irradiating means. The sample gas in theanalysis cell is ionized by the irradiation with light. The producedions from the sample gas are irradiated with a high frequency having anirradiation frequency near the ion cyclotron resonance frequency,whereby the ions cause ion cyclotron resonance. The ion cyclotronresonance is detected as a high-frequency decaying signal, converted toa digital signal, Fourier transformed and stored in the memory means asa mass spectrum.

If the sample gas contains a specific component to be measured and othercomponents and the components other than said specific component have asufficiently high ionization potential, the mass spectrum of thespecific component to be measured can be obtained by irradiating a lighthaving a specific wavelength sufficient to excite molecules of thecomponent to be measured to the ionization potential of the component tobe measured, and, as a result, the separation and the determination ofthe component to be measured become possible. In this case, since acomponent having an ionization potential higher than that of thecomponent to be measured is not ionized, the mass spectrum of thiscomponent cannot be obtained. That unionized component is usuallyeliminated from the measuring system.

On the other hand, when a sample gas containing the component to bemeasured and other components having ionization potentials higher andlower than the ionization potential of the component to be measured issubjected to measurement, the sample gas is irradiated with the firstirradiation light having a wavelength which is sufficiently short forexciting molecules of the component to be measured to the ionizationpotential thereof. The resulting mass spectrum, i.e. the first massspectrum, includes mass spectra of other components having ionizationpotential lower than that of the component to be measured. The compositemass spectrum comprising a plurality of mass spectra is stored in thememory means. Furthermore, molecules of other components having anionization potential higher than the excited ions of the component to bemeasured are not ionized and usually eliminated from the measuringsystem. Then, the sample gas is irradiated with the second irradiationlight having a wavelength slightly different from the wavelength of theprevious irradiation light. This irradiation light excites the moleculeshaving lower ionization potential than that of the component to bemeasured, and the second mass spectrum corresponding to these componentsis obtained. This mass spectrum is also a composite mass spectrum of theexcited components. This composite mass spectrum is also stored in thememory means.

Then, the first mass spectrum and the second mass spectrum are calledout from the memory means and are subjected to subtraction processing bythe subtraction processing means. As a result of the subtractionprocessing, a difference spectrum is obtained. This difference spectrumis a mass spectrum of the component having an ionization potential equalto the slight energy gap between the first irradiation light and thesecond irradiation light. By appropriately selecting the wavelength ofthe first irradiation light and the second irradiation light, massspectra of any components equal in ionization potential in a sample gaswhich shows a complex composite spectrum can be selectively obtained.

In the apparatus for the analysis of mixed gas components of the presentinvention, luminous intensity of the light irradiated to the sample gasis optionally adjusted to diminish or remove fragment peaks from massspectrum peaks of the component to be measured, whereby a mass spectrumcomprising only the molecule peak of the component to be measured can beobtained and, thus, it becomes easy to identify or determine the massspectrum peaks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram which shows a Fourier transform ioncyclotron resonance mass spectrometer as one embodiment of the presentinvention.

FIG. 2 is a illustration of one example of an analysis cell applied tothe Fourier transform ion cyclotron resonance mass spectrometer which isone embodiment of the present invention.

FIG. 3 is a schematic diagram of a light irradiating device in theFourier transform ion cyclotron resonance mass spectrometer which is oneembodiment of the present invention.

FIG. 4 is a timing chart which shows the change in potential of therespective electrodes constituting the analysis cell.

FIG. 5 is a front view of a vacuum chamber in the Fourier transform ioncyclotron resonance mass spectrometer which is one embodiment of thepresent invention.

FIG. 6 is a graph which shows relation between ionization potentials ofthe respective molecules and molecular weight.

FIG. 7 shows a mass spectrum obtained when a sample gas was irradiatedwith a light having an irradiation power of high or low using theFourier transform ion cyclotron resonance mass spectrometer which is oneembodiment of the present invention. means will be explained.

FIG. 1 is a general circuit block diagram showing the Fourier transformmass spectrometric means which is one embodiment of the presentinvention.

As shown in FIG. 1, Fourier transform mass spectrometric means 1 has ahigh vacuum means which is not shown in FIG. 1 containing analysis cell2 in which the molecules of components constituting a sample gas areionized, a magnetic field applying means having permanent magnet 3, alight irradiating device 40, which is not shown in FIG. 1, but shown inFIG. 3, being a variable wavelength light irradiating means whichirradiates the sample gas in the analysis cell 2 with a light and whichcan continuously vary the wavelength of the light to be irradiated tothe sample gas, an electronic circuit device provided withhigh-frequency transmitting means 4 and resonance signal detecting means7, a controlling-operating means provided with control circuit 6 whichcarries out control of high-frequency pulse system relating to ioncyclotron resonance by the instructions from computer 27 and control ofthe light irradiating device 40 and operation controlling means 8,keyboard 9 and CRT display 10.

The high vacuum means has vacuum chamber 11, analysis cell 2 andintroduction pipe 34 as shown in FIG. 5.

Vacuum evacuation pump 32, being employed as an evacuation means isfitted to one end of the vacuum chamber 11. Vacuum detector 33 isprovided at the side wall of the vacuum chamber 11, and degree of vacuumin this vacuum chamber can be measured by the vacuum detector 33.

To another end of the vacuum chamber 11 is connected a sample gassupplying pipe 35 for supplying a sample gas into the analysis cell 2,and opening of this sample gas supplying pipe 35 opens at the anotherend face of the vacuum chamber 11. This vacuum chamber 11 is alwaysevacuated by the vacuum evacuation pump 32 to maintain a high vacuum ofbetter than 10⁻⁷ Pa.

The analysis cell 2 is disposed in the vacuum chamber 11. In thisexample, especially, the analysis cell 2 is disposed at the positionwhich is inside the another end of the vacuum chamber 11 and is centerof static magnetic field provided by permanent magnet 3.

The analysis cell 2 has a hexahedral cell 2A as shown in FIG. 2. As thehexahedral cell 2A, there may be used a cubic cell comprising a pair ofelectrodes perpendicular to the direction of magnetic field of thepermanent magnet 3, a pair of irradiating electrodes parallel to themagnetic field and perpendicular to each other, and a pair of receivingelectrodes. Such cubic cells may include conventional cells as describedin M. B. Comisarow, "Cubic Trapped Ion Cell for Ion CyclotronResonance", Int. J. Mass Spect. Ion Phys., 37, (1981), p. 251-257, andothers.

In this example, the hexahedral cell 2A has three pairs of electrodes,namely, a pair of receiving electrodes R, R', a pair of trap electrodesP, P' and a pair of irradiation electrodes T, T' as shown in FIG. 2.

As shown in FIG. 2, in this hexahedral cell 2A, a slight positivepotential, for example, 0.1-2 V is applied to a pair of the trapelectrodes P, P' disposed in perpendicular to the direction of magneticfield in order to prevent ions in the analysis cell 2 from drifting inthe direction of magnetic axis. The irradiation electrodes T, T' arearranged opposite to each other and along the direction of the magneticfield between a pair of the trap electrodes P, P', so that ahigh-frequency signal which excites ion cyclotron resonance is appliedto the ions generated in the hexahedral cell 2A for a short time, forexample, 0.1-10 ms. The receiving electrodes R, R' are arranged opposingto each other and along the direction of the magnetic field and inperpendicular to the trap electrodes P, P' and the irradiationelectrodes T, T', and they receive a high-frequency signal voltageinduced by the ion cyclotron resonance.

The permanent magnet 3 which is a part of the magnetic field applyingmeans has a pair of magnetic pole pieces 3a, 3b arranged opposing toeach other with interposing the analysis cell 2 therebetween.

As shown in FIG. 3, the variable wavelength light irradiating device 40is provided with excimer laser 41 which is a light emitting means,variable wavelength laser 44, frequency doubler device 45 and reflectivemirror 42, and a pulse laser beam emitted from the excimer laser emitter41 can be pumped by the variable wavelength laser 44, this laser beamcan be converted to a laser beam processed by the frequency doublerdevice 45, and this laser beam can be guided into the hexahedral cell 2Athrough the reflective mirror 42 and the viewport 43.

In this example, at least the emission timing and the intensity of thelaser beam emitted from the excimer laser emitter 41 which is a lightemitting means are controlled by the control circuit 6. A dye laser isused as the variable wavelength laser 44. The pulse laser beam emittedfrom the excimer laser emitter 41 is swept from 320 to 950 nm by thevariable wavelength laser 44.

The control circuit 6 outputs control signals to high-frequency emitter4a and high-frequency transmitter 4b under the computer control so thatthe various operations prescribed in the Fourier transform method, suchas ionization, high-frequency irradiation, measurement, quenching ofresidual ions and the like can be actuated in accordance with thespecified order.

FIG. 4 shows one example of a typical relation between the appliedvoltage of each electrode of the analysis cell and the induced signal inthe analysis period.

(a) First, the sample gas molecule is ionized by the pulsehigh-frequency laser beam irradiated to the inside of the cell by lightirradiating device 40.

(b) After irradiation of the high-frequency laser beam, the output gateof the high-frequency transmitter 4b opens after a previously determinedlapse of time.

(c) An irradiation frequency which is a high-frequency pulse is appliedto the irradiating electrodes T, T' of analysis cell 2 from thehigh-frequency emitter 4a.

(d) Ions excited by the irradiation frequency induce ion cyclotronresonance. After the ions are excited, the output gate is closed.

(e) Thus, signal of ion cyclotron resonance is induced at the receivingelectrodes R, R'.

(f) After the ion cyclotron resonance signal is received by thereceiving electrodes R, R' and just before the next measuring period, apositive potential and a negative potential are respectively given tothe trap electrode pair P, P' disposed in perpendicular to the magneticaxis, and the ions remaining in the analysis cell 2 are quenched.

The control circuit 6 is a circuit to control the respective electrodevoltages in the analysis cell 2, which performs the function toirradiate the laser beam to the sample molecule introduced into theanalysis cell 2 to ionize the molecule prior to the application of thehigh-frequency pulse upon receiving instructions from computer 27, thefunction to intercept the irradiation with laser beam at the time ofapplication of the high-frequency pulse and during the period ofmeasurement of the ion cyclotron resonance signal, and the function toquench the remaining ion at the termination of the measurement.

The resonance signal detecting means 7 is provided with pre-amplifier20, high-frequency amplifier 21, low-pass filter 22, and A/D converter23 which carries out a high-speed processing.

The pre-amplifier 20 amplifies individually the ion cyclotron resonancesignals induced to receiving electrodes R, R' in the analysis cell 2 andthen outputs them to the high-frequency amplifier 21.

The high-frequency amplifier 21 includes a frequency mixer not shown inFIG. 1. That is, it carries out mixing processing of the amplified ioncyclotron resonance signal and a reference signal of frequency f₀ whichis separately input from the operation controlling means 8 and convertsthe ion cyclotron resonance high-frequency signal to a low-frequencysignal of a difference frequency between the signal frequency and thefrequency f₀ and transmits the low-frequency signal to the low-passfilter 22.

The conversion of the frequency is carried out by holding the amplifiedinformation of signal waves and converting only the frequency to thedifference frequency between the signal frequency and the referencefrequency by the same method as so-called hetrodyne detection incommunication equipments. The reference frequency f₀ is preferably sethigher than the ion cyclotron resonance frequency.

The low-pass filter 22 eliminates folding-over signals at the time ofthe A/D conversion by the A/D converter 23, and the cut off frequency isset in advance to be 1/2 or lower of the clock frequency of the A/Dconverter 23.

The A/D converter 23 converts the resonance signal from whichunnecessary frequency band is eliminated and which is simultaneouslyamplified to the signal level to such an extent as being A/Dconvertible, to the digital signal and outputs the digital signal to theoperation controlling means 8.

The operation controlling means 8 comprises a computer 27 which carriesout control of the whole system and operations such as subtractionprocessing, a memory device 28, an output device 29, and an interface 30which controls the A/D converter 23 and, furthermore, receives theoutput from the A/D converter 23 at a high speed and transmits a controlsignal from the computer 27 to the control circuit 6.

The resonance signal from which the unnecessary frequency band has beeneliminated and which has been amplified to the signal level adequate tothe A/D converter 23 is converted to a digital signal by the A/Dconverter 23, transferred to the computer 27 through the interface 30,and stored in the memory device 28 as a time-domain datum. After themeasurement, the time-domain datum is subjected to high-speed Fouriertransform processing by the computer 27 and converted tofrequency-domain datum, namely, a normal mass spectrum, which is againstored in the memory device 28. This memory device 28 has at least thefirst memory not shown in FIG. 1 which stores the first mass spectrumobtained by irradiation with a laser beam of a specific wavelength andthe second memory not shown in FIG. 1 which stores the second massspectrum obtained by irradiation with a laser beam having a wavelengthslightly different from the specific wavelength of the above laser beam.

These measurement control operations are all automatically performed bythe control signal from the computer 27 through the interface 30.

Next, the action of the above-mentioned devices will be explained.

A sample gas is introduced into analysis cell 2 from sample gassupplying pipe 35 through introduction pipe 34. In this example, sincethe introduction pipe 34 is provided between a sample introduction portof vacuum chamber 11 and the analysis cell 2, the whole of the samplegas is introduced into the analysis cell 2, thereby to inhibit thesample gas from diffusing in the vacuum chamber 11 without entering intothe analysis cell 2. As a result, especially, when the amount of thesample gas is slight, the sample gas can be efficiently subjected tomeasurement and the measuring sensitivity in mass spectrometric analysisis improved.

The introduced sample gas is ionized upon irradiation with the laserbeam emitted from light irradiating device 40. In more detail, in thelight irradiating device 40, a laser beam in the form of pulse isemitted from excimer laser emitter 41 and wavelength variable laser 41which is a pumping device emits a laser beam of 320-950 nm inwavelength. Since this wavelength is still not sufficiently short, alaser beam having a frequency which is twice higher than the frequencyof the above laser beam is formed by frequency doubler device 45, andthe sample gas in the analysis cell is irradiated with this laser beam.In the sample gas irradiated with the laser beam, only the moleculeshaving an energy level of the laser beam which satisfies the conditionsof multi-photon ionization strongly absorb the energy and are ionized.The molecules which do not satisfy the conditions of the multi-photonionization are not ionized but removed from the vacuum chamber 11 by thevacuum evacuation pump 32.

The ionized specific component in the sample gas is retained in theanalysis cell 2 by applying a voltage to the trap electrodes P, P', andthe ions do rotational movement in a plane perpendicular to the magneticfield due to the interaction between the electric charge possessed bythe ionized specific component and the static magnetic field applied tothe analysis cell 2.

For this rotational movement of ions, application of a high-frequencyfield from the irradiation electrodes T, T' in the analysis cell 2, ionswhich are in ion cyclotron resonance to result in uniform phases, formion packet and increase in radius of the rotation.

Even after cutting off the high-frequency voltage, ions of the ionizedspecific molecule continue the rotational movement. The ions graduallylose kinetic energy owing to collision with the remaining gas moleculesand diminish. Such decaying rotational movement of ions induces decayingvibration signal in the receiving electrodes R, R'. The frequency of thesignal equals to the rotational frequency of ions and the amplitude isproportional to the number of the ions.

The induced decaying signals are successively transmitted to theoperation controlling means 8 through resonance signal detecting means7.

The transmitted decaying signals are converted to digital signals, andafter converted, they undergo Fourier transformation by the computer anda mass spectrum can be obtained as a frequency component. The resultingmass spectrum is once stored in the memory device 28.

When the component ionized by the energy of the light having a specificwavelength irradiated by the light irradiating device 40 comprises asingle species, the component can be identified from the resulting massspectrum. Furthermore, determination of the quantity of the componentcan be performed from the peak area of the mass spectrum and others.

When the component ionized by the energy of the light having a specificwavelength irradiated by the light irradiating device 40 comprises aplurality of species, the mass spectrum obtained by the first laser beamirradiation, i.e. the first mass spectrum, is a mixed mass spectrumcomposed of mass spectra of a plurality of the components. Thiscomposite mass spectrum is stored in the first memory in the memorydevice 28.

Then, the second laser beam having a wavelength slightly different fromthat of the first laser beam is irradiated to the sample gas in theanalysis cell 2. Only the component wherein the energy level of thesecond laser beam satisfies the conditions of multi-photon ionizationthereof strongly absorbs the energy of said laser beam and are ionized.This ion is excited in the same manner as in irradiating the first laserbeam to cause ion cyclotron resonance and a mass spectrum is obtained.This mass spectrum is a mixed spectrum composed of mass spectra of aplurality of the components. In this example, this mass spectrum iscalled the second mass spectrum. The second masspectrum is once storedin the second memory in the memory device 28.

The first mass spectrum stored in the first memory and the second massspectrum stored in the second memory are read out and subjected tosubtraction processing by the computer 27. A difference spectrum isobtained by the subtraction processing. It becomes possible by thisdifference spectrum to carry out identification and determination of thespecific component having an ionization potential between the energylevel of the first laser beam and the energy level of the second laserbeam. The identification and determination of a specific molecule willbe explained below more specifically.

When the sample gas is a mixed gas of the components equal in molecularweight, for example, 1-butene, isobutene and t-2-butene, three massspectra are obtained by irradiation with lights having differentwavelength since they differ in ionization potential, and mass spectrumfor each molecule is obtained by the subtraction processing, and, thus,identification and quantitation become possible.

As explained above, in the present invention including this example,energy level of the light firstly irradiated to the sample gas is set atsuch level as including the ranges of ionization potential of allcomponents in the sample gas to be analyzed, and a mass spectrumobtained by irradiating the sample gas with this light having themaximum energy level is stored in the memory device as the first massspectrum. Then, another mass spectrum obtained by irradiating the samplegas with a light having an energy level slightly lower than the aboveenergy level is stored in the memory device as the second mass spectrum.When the second mass spectrum is subtracted from the first massspectrum, the resulting difference spectrum is a mass spectrum of thecomponent having an ionization potential present between the energylevel of the first irradiated light and that of the second irradiatedlight. Similarly, the third mass spectrum is obtained by irradiating thesample gas with a light having an energy slightly smaller than that ofthe second irradiated light. The above second mass spectrum is used asthe first mass spectrum of the present invention and the above thirdmass spectrum is used as the second mass spectrum of the presentinvention, and the second mass spectrum is subtracted from the firstmass spectrum to obtain a difference spectrum. This difference spectrumis a mass spectrum of the component having an ionization potentialpresent between the energy level of the second irradiated light and thatof the third irradiated light.

In this way, by varying the energy level of the light irradiated to thesample gas, in other words, by varying the wavelength of the lightirradiated to the sample gas, a mass spectrum is obtained for everyirradiation of light and the resulting spectra are subjected tosubtraction processing and the wavelength of the light to be varied isproperly selected, whereby mass spectra of the components equal in mass,but different in ionization potential can be separated. As a result, forexample, separation of the mass spectra of the respective isomers canalso be performed, and, thus, identification of a specific component canbe performed. Moreover, when mass spectrum of the specific component isobtained, it also becomes possible to determine the quantity of thespecific component in the sample gas by calculating the area of thespectrum peak. The wavelength of the light which is varied and employedfor every irradiation is suitably determined depending on the componentscontained in the sample gas.

On the other hand, when the sample gas is, for example, an exhaust gasfrom automobiles, as the molecules ionized by irradiating a light of thewavelength which provides an ionization potential of 9 eV, for example,molecules of 1,3-butadiene and t-2-butene are ionized and a mixed massspectrum based on these two kinds of molecules is obtained. The massnumber of 1,3-butadiene is 54 and that of t-2-butene is 56. Therefore,when the power of light irradiated by the wavelength variable lightirradiating device is reduced to carry out a soft ionization, theinterference with fragment ions can be avoided and the ions derived fromboth of the two compounds can be separated and determined as molecularpeaks differing in mass number.

FIG. 7 shows a mass spectrum of a sample. It can be easily understoodfrom FIG. 7 that when a sample is irradiated with a laser beam having anappropriate wavelength at a high intensity, a mass spectrum having manyfragment peaks is obtained, and when the sample gas is irradiated with alaser beam having an equal wavelength at a low intensity, a massspectrum having substantially no fragment peaks and having emphasizedmolecular peak is obtained.

Accordingly, in the present invention including the apparatus of thisexample, a mass spectrum having separated molecule peaks for a pluralityof molecules having the same ionization potential but differing inmolecular weight can be obtained by lowering the intensity of laser beampower from the wavelength variable light irradiating device or theintensity of light irradiated from the wavelength variable lightirradiating means.

An example of the present invention has been explained hereinabove, butthis example is an illustration of the present invention. It is needlessto say that the present invention can be applied to analysis of othermixed gases without departing from the scope of the invention.

The present invention can provide an apparatus for the analysis of mixedgas components according to which even when the sample gas contains aplurality of components having the same molecular weight but differingin ionization potential, mass spectrum of each component can be obtainedseparately, and identification and determination of the respectivecomponents can be performed in high precision.

The present invention can provide an apparatus for the analysis of mixedgas components according to which even when the sample gas contains aplurality of components having the same or substantially the sameionization potential, but differing in molecular weight, mass spectrumof each component can be obtained separately, and identification anddetermination of the respective components can be performed in highprecision.

The present invention has solved not only the problem of complexity ofusing, in a row, a plurality of analytical instruments differing inprinciples, but also the problem of unexpected and inevitable errors inmeasurement as seen when interfering components are present.Furthermore, the present invention has solved the problem of requiring along measuring period of several minute to several ten minutes whencarrying out a pretreatment of separating the components by gaschromatography. Thus, the effect brought about by the present inventionis highly noticeable.

The apparatus for the analysis of mixed gas components of the presentinvention is particularly suitable in case when analysis of componentsof an exhaust gas of automobiles must be performed in a short time.

What is claimed is:
 1. An apparatus for the analysis of mixed gascomponents which comprises a Fourier transform mass spectrometric meansfor ionizing a sample gas introduced into a high vacuum cell disposed ina static magnetic field, applying to ions of a gas a high frequencyelectric field by applying a high frequency to a pair of irradiatingelectrodes disposed in the high vacuum cell to induce ion cyclotronresonance to the ions of a specific component to be measured, detectingthe ion cyclotron resonance as a high-frequency decaying electricsignal, converting the resulting high-frequency decaying electric signalto a digital signal, and converting the digital high-frequency decayingsignal which is a time-domain signal to a frequency-domain signal and awavelength variable light irradiating means for irradiating the samplegas with a light of a single wavelength to give an ionization energy tomolecules of the components constituting the sample gas, saidirradiating means being able to vary the wavelength of the irradiationlight.
 2. An apparatus according to claim 1, wherein the wavelengthvariable light irradiating means has a light source which outputs acoherent light of a wavelength of an energy level corresponding to theionization potential of the sample gas.
 3. An apparatus according toclaim 2, wherein the wavelength variable light irradiating means isconstructed so as to be able to continuously vary the irradiation lightintensity.
 4. An apparatus according to claim 1, wherein the wavelengthvariable light irradiating means is constructed so as to be able tocontinuously vary the irradiation light intensity.
 5. An apparatusaccording to claim 4 which has a subtraction processing means forsubjecting to subtraction processing a first mass spectrum detected byirradiating the sample gas with an irradiation light having a givenwavelength and a second mass spectrum detected by irradiating the samplegas with an irradiation light having a wavelength differing from saidgiven wavelength.
 6. An apparatus according to claim 1 which comprises asubtraction processing means for subjecting to subtraction processing afirst mass spectrum detected by irradiating the sample gas with anirradiation light having a given wavelength and a second mass spectrumdetected by irradiating the sample gas with an irradiation light havinga wavelength differing from said given wavelength.
 7. An apparatus foranalyzing components of a gas mixture by Fourier transform ion cyclotronresonance mass spectrometry, the apparatus including a vacuum cell,means for creating a static magnetic field within the cell, means forcreating a high-frequency electric field within the cell to induce ioncyclotron resonance, means for detecting a high-frequency decayingelectric signal, Fourier means for converting the electric signal into afrequency-domain signal, and ionization means for ionizing the gasmixture within the cell; the improvement whereinthe ionization meansincludes means for irradiating the gas mixture with monochromatic lightof a predetermined wavelength; whereby ions created in the gas mixturehave an ionization energy less than or equal to a photon energy of themonochromatic light, such that ionization of the components isselective.
 8. The apparatus according to claim 7, wherein the wavelengthof the monochromatic light is variable, whereby the ionization energy islikewise variable.
 9. The apparatus according to claim 7, wherein theFourier means includes means for taking a difference between a firstfrequency-domain signal from a first light wavelength and a secondfrequency-domain signal from a second light wavelength different fromthe first light wavelength.